1. Field of the Invention
The present invention relates to an ultrasonic machining method for mirror-finishing (polishing) the surface of a workpiece made of boride cermet or Ni(12.5)-Cr(20.5) rolled material, said workpiece having hard layers and comparatively soft metal binding layers mixed together.
2. Description of the Prior Art
Ultrasonic machining for polishing is usually accomplished by the impact action of hard abrasive grains placed between a workpiece and a tool attached to a vibrating horn of an ultrasonic machine. The impact action is induced by the ultrasonic vibration of the tool. The abrasive grains, which are made of meta-B.sub.4 C or CBN, bring about minute brittle fracture in the work surface of the workpiece. In other words, ultrasonic machining resorts on the mechanical removal of material from a workpiece which is achieved by the direct application of mechanical force (machining stress) to a workpiece. Therefore, the machining precision of the finished surface is no better than that achieved by mechanical machining which also has a limitation.
In order to improve the precision of finishing, there was devised and proposed an ultrasonic machining method combined with electrochemical machining. According to this method, an electrolyte containing hard abrasive grains is placed between a workpiece and a tool and an electric current is applied across them for electrochemical machining. (See Japanese Patent Laid-open No. 282821/1987.)
This new ultrasonic machining method combined with electrochemical machining provides a higher finish precision than the conventional ultrasonic machining. Nevertheless, it still has a disadvantage that it leaves strain in the workpiece because it achieves machining by the impact of hard abrasive grains against the workpiece or by the mechanical force directly applied to the workpiece. Therefore, it is not yet satisfactory for mirror finishing which needs an extremely high precision.
There is known a machining method for mirror finish which is a combination of electrolytic machining and grinding with abrasive grains. Grinding is intended to remove the passive oxide film which is formed by electrolysis on the work surface of the workpiece as electrolysis proceeds (for the removal of material by chemical process). (Refer to Japanese Patent Laid-open No. 1395/1978.) This conventional machining process is carried out with an apparatus consisting of a workpiece holder and a tool which faces to the workpiece and slides parallel to the work surface of the workpiece. The tool is provided with electrodes and buffs arranged alternately. The buffs press and rub abrasive grains against the work surface. To achieve electrolytic machining, an electrolyte containing irregularly shaped hard abrasive grains suspended therein is passed through the gap between the tool and the work surface. Electrolytic machining forms a film on the work surface, and this film is removed by buffing. Thus buffing promotes electrolytic machining and permits the minute projections on the work surface to undergo electrolytic machining preferentially. In this way, the work surface undergoes mirror finishing.
A disadvantage of this machining method is that the hard abrasive grains suspended in the electrolyte cause strain to the work surface when they remove the film formed on the work surface as the result of electrolysis. One reason for this disadvantage is that it is difficult to control the pressure of the tool according to the thickness of the film to be removed because the tool presses hard abrasive grains against the work surface to remove the film. Another reason is that the abrasive grains are irregularly shaped and are made of hard material so that they apply a mechanical force (machining stress) to the workpiece.
Because of these disadvantages, the conventional machining method does not meet the requirements for mirror finish with an extremely high precision which have arisen with the recent technological development. For example, the cavity of a die cast mold, into which a high-temperature molten metal is injected, should have a mirror finish of extremely high precision so that it gives a molded item having a good surface and a high dimensional accuracy.